Production process for highly water absorbable polymer

ABSTRACT

Disclosed is a process for preparing water absorbing, cross-linked acrylate resins by aqueous polymerization of (A) acrylic acid neutralized 70 to 100 mole percent for example with ammonia, and/or caustic alkali and/or an amine; with (B) acrylamide in a mole ratio of 70 to 100 mole percent (A) to 30:0 mole percent (B); and (C) a water miscible or a water soluble polyvinyl monomer in an amount of 0.001 to 0.3 weight percent based on the total weight of (A) and (B). In accordance with one embodiment of the invention, the concentration of monomers (A) plus (B) should be at least 70 percent by weight of the polymerization mixture of (A) plus (B) plus (C) to achieve a substantially dry polymer (less than 15 weight percent water) when polymerization is completed by utilizing the exothermic heat of polymerization and cross-linking to drive off water without the need for additional heating to obtain a dry solid. The addition of polystyrene and/or methylcellulose substantially increases the water absorbing capacity of these polymers.

FIELD OF THE INVENTION

The present invention relates to a method of manufacturing polyacrylateresins having improved water absorbing properties and more particularlyto an improved process of preparing cross-linked polymers of acrylicacid, acrylamide and polyvinyl monomers.

BACKGROUND OF THE INVENTION

Water absorbing resins have found wide use in sanitary goods, hygenicgoods, water retaining agents, dehydrating agents, sludge coagulants,thickening agents, condensation preventing agents and release controlagents for various chemicals. Water absorbing resins heretofore knowninclude hydrolysis products of starch-acrylonitrile graft polymers,carboxymethylcellulose, cross-linked polyacrylate products and otherresins such as polyvinyl alcohol, polyethylene oxide andpolyacrylonitrile resins. Of these water absorbing resins, thehydrolysis products of starch and acrylonitrile graft polymers havecomparatively high ability to absorb water but require a cumbersomeprocess for production and have the drawbacks of low heat resistance anddecaying or decomposing easily due to the presence of starch.

One of the processes for polymerizing acrylic acid and acrylates isaqueous solution polymerization. The polymer obtained by this process issoluble in water and, therefore, is cross-linked to modify the polymerinto a useful water absorbing resin. However, even if the modificationis effected by reacting a cross-linking agent concurrently with or afteraqueous solution polymerization, the resulting reaction product is inthe form of a highly viscous aqueous solution or a gel containingabsorbed water which is difficult to handle. Thus, the aqueous solutionor gel must be dehydrated (dried) to obtain a water absorbing resin inthe desired solid or powder form. It is nevertheless difficult to drythe reaction product efficiently by the usual rotary drum roller methodor spray drying method because care must be taken to avoid excessivecross-linking which results from overheating during drying andinsufficient drying results in reduced cross-linking density. Extremedifficulties are therefore encountered in preparing a product of adesired low water content and good water absorbing ability.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a process for preparinga water absorbing cross-linked acrylate resin of low water content byaqueous solution polymerization without any additional dehydrating ordrying step.

Another object of the present invention is to provide a process forpreparing a cross-linked polyacrylate resin by co-polymerization ofacrylic acid neutralized 70-90 mole percent, with acrylamide and apolyvinyl monomer in proportions of 0 to 30 mole percent acrylamide and70-100 mole percent partially neutralized acrylic acid.

Another object of the present invention is to provide a process forproducing a polyacrylate resin cross-linked with 50-200 parts permillion, based on the weight of monomers, of a water miscible or watersoluble polyvinyl cross-linking agent.

Still another object of the present invention is to provide a method ofmanufacturing a cross-linked polyacrylate resin polymerized in aqueoussolution having a monomer concentration of at least 70 weight percent sothat the heat of polymerization and cross-linking drives off the waterto provide a dry (less than 15% by weight water), solid resin.

In brief, the present invention is directed to a process for preparingwater absorbing, cross-linked acrylate resins by aqueous polymerizationof (A) acrylic acid neutralized 70 to 100 mole percent for example withammonia, and/or caustic alkali and/or an amine; with (B) acrylamide in amole ratio of 70 to 100 mole percent (A) to 30:0 mole percent (B); and(C) a water miscible or a water soluble polyvinyl monomer in an amountof 0.001 to 0.3 weight percent based on the total weight of (A) and (B).

In accordance with one important embodiment of the present invention,the concentration of monomers (A) plus (B) should be at least 70 percentby weight of the polymerization mixture of (A) plus (B) plus (C) toachieve a substantially dry polymer (less than 15 weight percent water)when polymerization is completed by utilizing the exothermic heat ofpolymerization and cross-linking to drive off water without the need foradditional heating to obtain a dry solid.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with one embodiment of the present invention across-linked polyacrylate resin is prepared by aqueous solutionpolymerization while dehydrating or drying the reaction product duringpolymerization by utilizing the exothermic heat from the polymerizationand cross-linking reactions for drying.

It has been found that acrylic acid neutralized in the range of 70 to100 mole percent will polymerize rapidly with acrylamide and cross-linkrapidly with a polyvinyl monomer cross-linking agent to drive awayexcess water leaving a solid water absorbing resin having a desireddegree of polymerization as well as new and unexpected water absorbingcapacity. One or more polymerization catalysts or initiators can beadded to the aqueous monomer mixture to aid in polymerization.

Acrylic acid neutralized 70-100 mole percent is mixed with awater-miscible or water-soluble polyvinyl monomer in an aqueous solutionat a temperature of about 20° to 100° C. The solution is subjected to apolymerization reaction and a cross-linking reaction by the addition ofa polymerization initiator. The polymerization reaction proceedssufficiently within a very short period of time and if the monomerconcentration is at least 70 percent by weight of the aqueous monomermixture, the heat of the polymerization and cross-linking reactions willevaporate water rapidly from the reaction system to form a dry solid(less than 15 percent by weight water) water absorbing resin without theneed for any subsequent drying step. The solid can be easily pulverizedinto a powder suitable for any desired use.

Further, according to one embodiment of the invention, when the monomerconcentration is at least 70 percent by weight of the polymerizationmixture, the polymerization reaction and evaporation of water can becompleted very rapidly, usually within several minutes, without externalheating. The process can be practiced very efficiently with greatlyreduced consumption of heat energy while achieving a desired degree ofpolymerization very suitable for a commercial operation.

According to the process of the invention, a hot, i.e. at least 25° C.,aqueous solution is prepared first including acrylic acid neutralized 70to 100 mole percent, acrylamide, a water-miscible or water-solublepolyvinyl monomer, and water. The aqueous solution can be preparedeasily by placing (A) acrylic acid, and an amine, and/or a causticalkali and/or ammonia for neutralizing the acid; (B) acrylamide; and (C)a polyvinyl monomer into water to form a mixed monomer solution. Inaccordance with one important embodiment of the present invention, themonomers comprise at least 70 percent by weight of the mixed solution toprovide a self drying mixture upon polymerization and cross-linking. Todissolve the monomers thoroughly, the mixture can be heated to anelevated temperature up to the boiling point of water i.e. 100° C.

Others who have attempted aqueous polymerization of acrylic monomershave found that potassium neutralization of acrylic acid is the only wayto obtain an acrylate polymer having sufficient water absorbing capacityand have found that potassium acrylate is necessary to achieve apolymerizable monomer mixture of sufficiently high concentration toachieve self drying. In accordance with the present invention, it hasbeen found that by including acrylamide in the monomer mixture, theacrylic acid can be neutralized with a substantial number ofneutralizing agents, such as ammonia or an amine, or any alkali metalcompound in addition to potassium hydroxide, such as a sodium hydroxide,lithium hydroxide, cesium hydroxide, potassium carbonate or sodiumcarbonate without difficulty in preparing an aqueous solution of highconcentration i.e. 70-90 weight percent. Further, the resulting waterabsorbing resins have unexpectedly high water absorbing capacity.Addition of the neutralizing agent in molar excess will not cause anyparticular problem, but the excess does not participate in thepolymerization reaction.

The polyvinyl monomer to be used in the invention should be misciblewith or soluble in water so that the monomer will be uniformly dissolvedor dispersed in the aqueous solution of the monomer mixture. Examples ofsuch polyvinyl monomers include bisacrylamides such asN,N'-methylenebisacrylamide and N,N'-methylenebismethacrylamide;polyacrylic (or polymethacrylic) acid esters represented by thefollowing formula (I); and diacrylamides represented by the followingformula (II). Among these, especially preferably areN,N'-methylenebisacrylamide, N,N'-methylenebismethacrylamide and likebisacrylamides. ##STR1## wherein X is ethylene, propylene, trimethylene,hexamethylene, 2-hydroxypropylene, (CH₂ CH₂ O)_(n) CH₂ O)_(n) CH₂ CH₂ --or ##STR2## n and m are each an integer of from 5 to 40, and k is 1 or2.

The compounds of the formula (I) are prepared by reacting polyols, suchas ethylene glycol, propylene glycol, trimethylolpropane,1,6-hexanediol, glycerin, pentaerythritol, polyethylene glycol andpolypropylene glycol, with acrylic acid or methacrylic acid. ##STR3##wherein l is 2 or 3.

The compounds of the formula (II) are obtained by reactingpolyalkylenepolyamines, such as diethylenetriamine andtriethylenetetramine, with acrylic acid.

The polyvinyl monomer is used in an amount of about 0.001 to 0.3 wt. %,preferably 0.005 to 0.1 wt. %, of the amount of acrylic monomers in theaqueous monomer mixture. If more than about 0.3 weight percent of thepolyvinyl monomer is used, the resulting polymer will have exceedinglyhigh strength when in the form of a gel on absorption of water andtherefore exhibit impaired water absorbing ability, whereas if it isused in an amount below about 0.001 weight percent, a polymer sol willbe obtained having low gel strength exhibiting reduced water absorbingability.

The aqueous mixed monomer solution is heated and thereafter subjected topolymerization and cross-linking reactions with the addition of apolymerization initiator. Although the temperature of the aqueous mixedmonomer solution is not particularly limited since the mixed monomersolution is initiated into polymerization by the addition of theinitiator, the temperature is usually about 50° to about 85° C.,preferably about 60° to about 75° C. Various polymerization initiatorsare usable which are known for use in preparing polyacrylates. Examplesof useful initiators are redox initiators comprising a reducing agent,such as a sulfite or bisulfite of an alkali metal, ammonium sulfite orammonium bisulfite, and an initiator, such as a persulfate of an alkalimetal or ammonium persulfate, in combination with the reducing agent;azo initiators including azobis-isobutyronitrile,4-t-butylazo-4'-cyanovaleric acid, 4,4'-azobis(4-cyanovaleric acid) and2,2'-azobis(2-amidinopropane)-hydrochloric acid salt; and the like.These initiators can be used singly or in a suitable combination. Ofthese, especially preferable are a redox initiator composed of ammoniumpersulfate and sodium hydrogensulfite, and azo initiators such aazobisisobutyronitrile and 2,2'-azobis(2-amidinopropane)-hydrochloricacid. These initiators are advantageously used usually in the form of anaqueous solution but can be used as diluted with a suitable solvent. Theinitiator is used in a usual amount, i.e. in an amount, calculated assolids, of about 0.1 to about 10%, preferably about 0.5 to about 5%, ofthe combined weight of the monomers, namely acrylate (and free acrylicacid), acrylamide, and polyvinyl monomer. Depending on the amount andkind of the initiator, the initiator is usable together with isopropylalcohol, alkylmercaptan or other chain transfer agents to control themolecular weight of the polyacrylate to be obtained.

By the addition of the polymerization initiator, the mixed monomersolution is subjected to polymerization with evaporation of waterwithout heating the system from outside. More advantageously, thereaction is carried out by admixing a predetermined amount of theinitiator or an aqueous solution thereof with the mixed monomer solutionand causing the resulting mixture to flow down onto and spread over atraveling conveyor belt. The initiator can be added to the mixed monomersolution as it is poured onto the conveyor belt.

The polymerization proceeds rapidly after admixing the initiator withthe mixed monomer solution and is completed within a short period oftime, usually in about 30 seconds to about 10 minutes. The reaction isexothermic, so that the reaction system is rapidly heated to about 100°to about 130° C. by the heat of polymerization. Consequently,particularly where the monomer concentration in the mixed solution is atleast 70 percent by weight, the water evaporates from the system rapidlyto give a relatively dry, solid polymer of low water content withoutresorting to any external heating. The water content of the polymer,when reacted at high (at least 70 weight percent) monomer concentration,is usually up to about 15%, and generally about 8 to 12% by weight.Subsequently, the dry solid polymer can be made into the desired powdereasily by a usual method, for example by pulverization, without a dryingstep.

In accordance with another important feature of the present invention,polystyrene and/or methylcellulose can be added to the mixed monomersolution in an amount of 0.5 to about 10 percent based on the totalweight of monomers in the mixed monomer solution to increase theporosity and water absorbing capacity of the polymers. It has beenfound, quite surprisingly, the polystyrene and methylcellulose willsubstantially increase the water absorbing capacity of the resindescribed herein. To achieve the full advantage of the presentinvention, the polystyrene and methylcellulose should be added in anaverage grain size of less than or equal to 5 micrometers. Examples 8-10and 14-16 to follow show the surprising increase in water absorptionwith the addition of polystyrene or methylcellulose.

The powder thus obtained has outstanding water absorbing ability and isuseful for sanitary goods, paper diaper, disposable diaper and likehygenic goods, agricultural or horticultural water retaining agents,industrial dehydrating agents, sludge coagulants, thickening agents,condensation preventing agents for building materials, release controlagents for chemicals and various other applications.

The present invention will be described in greater detail with referenceto the following examples.

EXAMPLE 1

28.1 gr. of acrylic acid and 11.9 gr. of acrylamide were dissolved in179.1 gr. of distilled water and then 10.9 gr. of NaOH was added for 70mole percent partial neutralization of acrylic acid; 0.003 gr. ofN,N'-methylenebisacrylamide was then added as the polyvinyl monomer. Inthis case, 0.04 gr. of 2,2'-azobis(2-amidinopropane)hydrochloride wasadded as the polymerization initiator and the initial temperature of themixed monomer solution was 50° C.

EXAMPLE 2

48.1 gr. of acrylic acid and 11.9 gr. of acrylamide were dissolved in159 gr. of distilled water and then 22.7 gr. of NaOH was added forpartial neutralization of acrylic acid in an amount of 85 mole percent.0.006 gr. of N,N'-methylenebisacrylamide was then added as the polyvinylmonomer. In addition, 0.048 gr. of ammonium persulfate and 0.048 gr. ofsodium hydrogen-sulfite were added as the polymerization initiators. Inthis case, the initial temperature of the mixed monomer solution was 40°C.

EXAMPLE 3

90.1 gr. of acrylic acid and 9.9 gr. of acrylamide were dissolved in118.8 gr. of distilled water and 52.6 gr. of KOH was added for 75 molepercent partial neutralization of acrylic acid. 0.018 gr. ofN,N'-methylenebisacrylamide was added as the polyvinyl monomer, and 0.08gr. of ammonium persulfate and 0.08 gr. of sodium hydrogen-sulfite wereadded as the polymerization initiators. The initial temperature of themixed monomer solution was 30° C.

EXAMPLE 4

48.1 gr. of acrylic acid and 11.9 gr. of acrylamide were dissolved in252.1 gr. of distilled water and 27.4 gr. of aqueous ammonia was addedfor 70 mole percent partial neutralization of acrylic acid. In thiscase, the concentration of the ammonia is 29 weight percent. Inaddition, 0.006 gr. of N,N'-methylenebisacrylamide was added as thepolyvinyl monomer. The polymerization was performed with the addition of0.048 gr. of ammonium persulfate and 0.048 gr. of sodiumhydrogen-sulfite for initiation. In this case, the initial temperatureof the mixed monomer solution was 30° C.

EXAMPLE 5

52.7 gr. of acrylic acid and 17.3 gr. of acrylamide were dissolved in 25gr. of distilled water and they were partially neutralized 80 molepercent with the addition of 32.8 gr. of KOH. 0.007 gr. ofN,N'-methylenebisacrylamide was added as to the polyvinyl monomer. Forthe polymerization catalyst 0.7 gr. of 2,2'-azobisisobutyronitriledissolved in 10 cc. of acetone was added. This solution was kept at 80°C. in a TEFLON coated, glass fiber reaction chamber until completion ofpolymerization and cross-linking reactions yielding a white, solidresin.

EXAMPLE 6

17.3 gr. of acrylamide was dissolved in 52.7 gr. of acrylic acid andpartial (70 mole percent) neutralization of acrylic acid wasaccomplished by the addition of 30 gr. of aqueous ammonia having aconcentration of 29 weight percent. In this case, for the polyvinylmonomer, 0.007 gr. of N,N'-methylenebisacrylamide was added and, as thecatalyst, 0.7 gr. of 2,2'-azobis(2-amidinopropane)hydrochloridedissolved in 8 gr. of distilled water was added. The polymerization wasstarted at 80° C. yielding a white, solid resin.

EXAMPLE 7

21 kg. of acrylic acid and 7 kg. of acrylamide were dissolved in 9.5 kg.of distilled water and the acrylic acid was partially neutralized with12 kg. of KOH. 0.036 kg. of methylenebisacrylamide as a polyvinylmonomer was added to provide an aqueous mixed monomer solution. Themixed monomer solution was mixed with 0.28 kg. of2,2'-azobisisobutyronitrile, dissolved in 2 kg. of acetone as apolymerization initiator. This mixture at a temperature of 60° C. wastransferred on an endless belt (600-700 mm. in width, and 7 m. inlength) at a thickness of about 1 cm. The polymerization was initiatedpromptly on the belt resulting in a white, solid resin.

EXAMPLE 8

The polymers of Examples 1-4, were dehydrated with acetone, dried andpulverized into powders. The polymers of Examples 5-7, were pulverizedwithout any drying into highly water absorbable powders. The products ofExamples 1-7, were compared to commercial prior art products. Thetesting method was as follows:

1 gr. of sample was added to 1 liter of distilled water with agitation.After one hour of settling for water absorption, the water-polymer wasfiltered through a 100 mesh sieve and the amount of water absorbeddetermined by measuring the volume of filtrate recovered. In addition,parallelly, 5 gr. of the polymers of Examples 1-7 were added to 1 literof 1 percent NaCl solution, and testing was conducted the same as in thecase of distilled water. The results of the testing are shown in TableI. Water absorbability is shown as amount of absorbed water/weight ofresin.

Although any alkaline metal and any amine can be used for neutralizationof the acrylic acid, the ammonium acrylate has been found to have thebest solubility in water so that a higher concentration of the ammoniumacrylate salt in water is possible. Further, the polymer manufacturedwith the ammonium acrylate salt is superior in water absorption.Further, it has been found that the incorporation of acrylamide in thepolymerization process provides a polymer having better solubility inwater and better stability in the manufacturing process.

                  TABLE I                                                         ______________________________________                                        Examples 1-7                                                                            Distilled Water                                                                         1% NaCl Solution                                          ______________________________________                                        Example 1   650 times   68 times                                              Example 2   670 times   70 times                                              Example 3   680 times   72 times                                              Example 4   620 times   65 times                                              Example 5   540 times   57 times                                              Example 6   520 times   54 times                                              Example 7   540 times   56 times                                              Commercial  520 times   49 times                                              product (1)                                                                   Commercial  504 times   47 times                                              product (2)                                                                   ______________________________________                                    

EXAMPLE 8

52.7 gr. of acrylic acid and 17.3 gr. of acrylamide were dissolved in 20gr. of distilled water. 32.8 gr. of KOH was added for 80 mole percentpartial neutralization of acrylic acid. Then 0.007 gr. ofN,N'-methylenebisacrylamide polyvinyl monomer was added with 2.5 gr. ofmethylcellulose. The aqueous monomer mixture was homogenized withagitation. Next, as an initiator, 0.7 gr. of 2,2'-azobisisobutyronitriledissolved in 10 cc of acetone was added. This mixed solution was kept at80° C. in a water bath surrounding the reaction chamber. Polymerizationwas initiated as the temperature of the mixture increased as a result ofthe surrounding water bath, resulting in a white, solid porous resin.

EXAMPLE 9

20.8 gr. of acrylamide was dissolved in 49.2 gr. of acrylic acid and28.0 gr. of aqueous ammonia (29% concentration) was added for 70 molepercent neutralization of the acrylic acid. Next, 0.01 gr. ofN,N'-methylenebisacrylamide and 5 gr. of methylcellulose were added andstirred to homogenize. As an initiator, 0.7 gr. of2,2'-azobis(2-amidinopropane)-hydrochloride dissolved in 5 gr. ofdistilled water, was added. The polymerization was initiated in a TEFLONcoated glass reaction chamber kept at 80° C., as in Example 8 yielding awhite, solid porous resin having a water content of about 10% by weight.

EXAMPLE 10

70 gr. of acrylic acid was dissolved in 20 gr. of distilled water. 40.8gr. of KOH was added to neutralize 75 mole percent of the acrylic acid.Next, 0.007 gr. of N,N'-methylenebisacrylamide and 2.0 gr. ofmethylcellulose were added, and the mixture homogenized with agitation.Then, as an initiator, 0.7 gr. of 2,2'-azobisisobutyronitrile dissolvedin 10 cc. of acetone was added. This solution was transferred to thereaction chamber kept at 80° C. and polymerization was initiated with arise in temperature of the mixture resulting in a white, solid porousresin of low water content.

EXAMPLE 11

Same as Example No. 8, but having no methylcellulose.

EXAMPLE 12

Same as Example No. 9, but having no methylcellulose.

EXAMPLE 13

Same as Example No. 10, but having no methylcellulose.

The water absorbable resins of Example 8-13 were pulverized directlyinto powder without any drying process.

The water absorbing capacity of the resins of Examples 8-13 was testedas follows:

1 gr. of the resin powders (20-40 mesh) of Examples 8-13 were each addedto 2 liters of distilled water with agitation. After settling for 1hour, the water-polymers were filtered through a 100 mesh sieve and thevolume of filtrate measured to give the amount of water absorbed in theresin. In addition, 5 gr. of the resins of Examples 8-13 were each addedto 1 liter of 1% NaCl solution, and the testing was conducted the sameas the case of distilled water. The results are shown in Table II. Waterabsorbability is shown as amount of absorbed water/weight of resin

                  TABLE II                                                        ______________________________________                                        Examples 8-13                                                                           Distilled Water                                                                         1% NaCl Solution                                          ______________________________________                                        Example 8   1,300 times 98 times                                              Example 9   1,160 times 84 times                                              Example 10  1,360 times 102 times                                             Example 11    540 times 56 times                                              Example 12    470 times 50 times                                              Example 13    590 times 60 times                                              ______________________________________                                    

EXAMPLE 14

52.7 gr. of acrylic acid and 17.3 gr. of acrylamide were dissolved in 20gr. of distilled water. 32.8 gr. of potassium hydroxide was added toneutralize 80 mole percent of the acrylic acid. Next, 0.007 gr. ofN,N'-methylenebisacrylamide and 5 gr. of a polystyrene emulsion wereadded and homogenized under stirring. The polystyrene emulsion had aconcentration of 50 weight percent and a polystyrene grain size of about0.5 micro meters. Then, as the initiator, 0.7 gr. of2,2'-azobisisobutyronitrile dissolved in 10 cc. of acetone solution wasadded. The mixed monomer solution was kept at 80° C. in the reactionchamber immersed in a water bath. With the increasing temperature of themixed monomer solution polymerization was initiated resulting in awhite, solid, porous resin.

EXAMPLE 15

20.8 gr. of acrylamide was dissolved in 49.2 gr. of acrylic acid. 28.0gr. of aqueous ammonia (29% concentration) was added to neutralize 70mole percent of the acrylic acid. Then 0.01 gr. ofN,N'-methylenebisacrylamide was added. 3 gr. of the polystyrene emulsionof Example 14, but having a 1 micro meter average grain size, was addedand the mixed solution was homogenized with agitation. Next, as theinitiator, 0.7 gr. of 2,2'-azobis(2-amidinopropane)-hydrochloridedissolved in 5 gr. of distilled water was added. The solution was keptat 80° C. as in Example 14 and polymerization was initiated withincreased temperature resulting in a white, solid, porous resin.

EXAMPLE 16

70 gr. of acrylic acid was dissolved in 20 gr. of distilled water and40.8 gr. of KOH was added to neutralize 75 mole % of the acrylic acid.Next, 0.007 gr. of N,N'-methylenebisacrylamide was added. 2 gr. of thepolystyrene emulsion of Example 14 (but having an average grain size of5 micro meters), was added and the mixture homogenized. Next, as theinitiator, 0.7 gr. of 2,2'-azobisisobutyronitrile dissolved in 10 cc ofacetone was added. The mixed solution was kept at 80° C. in the waterbath to initiate the polymerization resulting in a white, solid, porousresin.

EXAMPLE 17

Same as Example 14, but having no polystyrene.

EXAMPLE 18

Same as Example 15, but having no polystyrene.

EXAMPLE 19

Same as Example 16, but having no polystyrene.

The polymers of Examples 14-19 were made into powdery resins without anydrying step.

The following tests were performed on the resins of Examples 14-19 todetermine water absorbing capacity:

1 gr. of the resins of Examples 14-19 (20-40-mesh) were each added into2 liter of distilled water with agitation and settling for one hour.After settling, a 100 mesh sieve was employed for filtration and theamount of absorbed water was calculated from the amount of filtrate.Their water absorption rate (absorbed water/weight of polymer) is shownin the following Table III.

                  TABLE III                                                       ______________________________________                                        Examples 14-19                                                                          Distilled Water                                                                         1% NaCl Solution                                          ______________________________________                                        Example 14  1,010 times 83 times                                              Example 15  960 times   76 times                                              Example 16  1,120 times 90 times                                              Example 17  540 times   56 times                                              Example 18  470 times   50 times                                              Example 19  590 times   60 times                                              ______________________________________                                    

We claim:
 1. A method of manufacturing a composition comprising mixing amonomer solution of (A) acrylic acid, neutralized 70 to 100 molepercent, (B) acrylamide in a mole ratio of (A):(B) in the range of 70:30to 90:10; (C) a water soluble or water miscible polyvinyl monomercross-linking agent in an amount of 0.001 to 0.3 percent by weight of(A) plus (B); and water to form a mixed monomer solution, wherein themonomers of the mixed monomer solution consist essentially of (A), (B)and (C) and the monomer concentration is at least 70 percent by weightof the monomer solution prior to polymerization initiation; andinitiating polymerization of monomers (A) and (B) such that duringpolymerization, the exothermic heat of reaction is substantially theonly heat energy used to accomplish polymerization, cross-linking and todrive off sufficient water to form a water absorbing cross-linkedpolyacrylate resin composition having a water content of 15 percent byweight or less.
 2. A method of manufacturing a composition comprisingmixing a monomer solution of (A) acrylic acid, neutralized 70 to 100mole percent, (B) acrylamide in a mole ratio of (A):(B) in the range of70:30 to 100:0; (C) a water soluble or water miscible polyvinyl monomercross-linking agent in an amount of 0.001 to 0.3 percent by weight of(A) plus (B); and water to form a mixed monomer solution, wherein themonomers of the mixed monomer solution consist essentially of (A), (B)and (C) and the monomer concentration is at least 70 percent by weightof the monomer solution prior to polymerization initiation; andinitiating polymerization of monomers (A) and (B) such that duringpolymerization, the exothermic heat of reaction is substantially theonly heat energy used to accomplish polymerization, cross-linking and todrive off sufficient water to form a water absorbing cross-linkedpolyacrylate resin composition having a water content of 15 percent byweight or less, and wherein said neutralized acrylic acid is one otherthan potassium acrylate.
 3. The method of claim 1 wherein the monomerconcentration in the mixed monomer solution is 70 to 90 weight percent.4. The method of claim 3 wherein the monomer concentration in the mixedmonomer solution is 70 to 80 weight percent.
 5. The method of claim 1wherein the polyvinyl cross-linking agent is included in the mixedmonomer solution in an amount of 0.005 to 0.05 percent by weight basedon the combined weight of (A) plus (B).
 6. The method of claim 5 whereinthe polyvinyl cross-linking agent is included in the mixed monomersolution in an amount of 0.005 to 0.02 percent by weight based on thecombined weight of (A) plus (B).
 7. The method of claim 1 wherein themixed monomer solution is heated to a temperature of 25° to 85° C. priorto polymerization initiation.
 8. A method of claim 1 wherein thepolyvinyl monomer is selected from the group consisting ofN,N'-methylenebisacrylamide and N,N'-methylenebismethacrylamide.
 9. Amethod of claim 1 wherein the polyvinyl monomer isN,N'-methylenebisacrylamide in an amount of 0.005 to 0.02 weight percentbased on the total weight of monomers in the mixed monomer solution. 10.The method of claim 1 wherein acrylic acid is neutralized with aneutralizing agent selected from ammonia, an amine, or a lithium,sodium, rubidium or cesium base.
 11. The method of claim 1 whereinpolymerization of said mixed monomer solution is initiated by adding apolymerization initiator to said mixed monomer solution.
 12. The methodof claim 11 wherein the monomer mixture has a temperature of 50° to 90°C. immediately prior to adding the polymerization initiator.
 13. Themethod of claim 11 wherein said polymerization initiator is added in anamount of at least 0.5 percent by total weight of monomers (A) plus (B)plus (C) in the mixed monomer solution.
 14. The method of claim 11including depositing said monomer mixture including said initiator ontoa support surface in sheet form for polymerization and cross-linking.15. A method of manufacturing a composition comprising mixing a monomersolution of (A) acrylic acid, neutralized 70 to 100 mole percent, (B)acrylamide in a mole ratio of acrylic acid:acrylamide in the range of70:30 to 90:10; (C) a water soluble or water miscible polyvinyl monomercross-linking agent in an amount of 0.001 to 0.3 percent by weight of(A) plus (B) and water to form a mixed monomer solution wherein themonomer concentration of (A) plus (B) is at least 70 percent by weightof (A) plus (B) plus (C) and wherein the reactant monomers (A), (B) and(C) of the mixed monomer solution comprise at least 70% by weight of themixed monomer solution; initiating polymerization of monomers (A) and(B) and polymerizing said monomers without substantial external heatingsuch that during polymerization, the exothermic heat of reaction issubstantially the only heat energy used to accomplish polymerization,cross-linking and to drive off sufficient water to form a waterabsorbing cross-linked polyacrylate resin composition having a watercontent of 15 percent or less as recovered from the mixed monomersolution.
 16. The method of claim 15 wherein the monomer concentrationin the mixed monomer solution is 70 to 90 weight percent.
 17. The methodof claim 16 wherein the monomer concentration in the mixed monomersolution is 70 to 80 weight percent.
 18. The method of claim 15 whereinthe polyvinyl cross-linking agent is included in the mixed monomersolution in an amount of 0.005 to 0.05 percent by weight based on thecombined weight of (A) plus (B).
 19. The method of claim 18 wherein thepolyvinyl cross-linking agent is included in the mixed monomer solutionin an amount of 0.005 to 0.02 percent by weight based on the combinedweight of (A) plus (B).
 20. The method of claim 15 wherein the mixedmonomer solution is heated to a temperature of 25° to 85° C. prior topolymerization initiation.
 21. The method of claim 15 wherein thepolyvinyl monomer is selected from the group consisting ofN,N'-methylenebisacrylamide and N,N'-methylenebismethacrylamide.
 22. Themethod of claim 15 wherein the polyvinyl monomer isN,N'-methylenebisacrylamide in an amount of 0.005 to 0.02 weight percentbased on the total weight of monomers in the mixed monomer solution. 23.The method of claim 15 wherein acrylic acid is neutralized with aneutralized agent selected from the group consisting of ammonia, anamine, and lithium, sodium, rubidium or a cesium base.
 24. The method ofclaim 15 wherein polymerization of said mixed monomer solution isinitiated by adding a polymerization initiator to said mixed monomersolution.
 25. The method of claim 24 wherein the monomer mixture has atemperature of 50° to 90° C. immediately prior to adding thepolymerization initiator.
 26. The method of claim 24 wherein saidpolymerization initiator is added in an amount of at least 0.5 percentby total weight of monomers (A) plus (B) plus (C) in the mixed monomersolution.
 27. The method of claim 20 wherein the water content of saidcross-linked polyacrylate resin is not greater than about 10% by weightas recovered from the mixed monomer solution after polymerization,without an additional drying step.